1. Field of the Invention
The present invention is directed to a system for measuring the water vapor content in drier ducts, ovens and the like. By measuring the sound speed in the medium and also in a reference tube containing dry air at the same temperature, a sensor which measures the water vapor independently of temperature is made possible. In addition, no calibrations are required, all the data being obtained by measurements of two time intervals which can be done to high accuracy. The sensor has virtually no temperature limit for all industrial applications.
2. Description of the Prior Art
Moisture measurements are necessary for the control of industrial processes where drying is involved. For example control of drying of paper in a paper mill requires a trouble-free rugged sensor for humidity control. Many industrial environments are harsh. High temperature, chemical corrosion or contamination are problems that make many types of humidity sensors unreliable in operation.
Polymer hygrometers of a particularly rugged type have been described by D. D. Lawson (U.S. Pat. No. 4,083,765) using a fluorocarbon polymer containing sulphonic acid groups. This polymer is called nafion and is made by DuPont. Its conductivity depends on water vapor content and temperature. A variation of this polymer with lithium doping has been shown to be immune to degradation from dust and organic vapors in a paper mill at temperatures near 65.degree. C. and air stream speeds of 40 ft/sec. (ref. 2). The sensor output is linear over a limited region of humidity and temperature.
Polymers which absorb water have been used as humidity sensors using a measurement of capacitance. This type is made by Rotronic Instrument Corporation (ref. 3).
Pacer systems offers a moisture sensor based on absorption of ultraviolet light by moisture (ref. 4). The probe is claimed to withstand 250.degree. C. Keeping the optical windows clean is difficult.
Dew Point Hygrometers are also used. Thermoelectric cooling of a surface makes dew settle on the surface whose property is detected by a change of light reflection, capacitance change (ref. 5) or attenuation of surface acoustic waves (ref. 6). Such sensors are not immune to the presence of containments such as dust and oil vapors.